Mythic levels

[Selion]

Im playing a long campaign, my party is currently level 12th; one of the main charachters of the story is a dragon demigod (they still never met him). This guy has obviously mythic levels, and he will give to the party a mission. I'm planning that he would use the 9th level mythic spell "ascension" to give them mythic tier 1 for a months (this may happen when they will be around level 14th-15th). If the quest will be accomplished their mythic rank will become permanent.
Someone has tried mythic levels in a mainly non mythic campaign? I fear that even one or two mythic ranks could make my party overpowered in respect to non mythics encounters, and i wouldn't give mythic tiers to every monster they will meet.

[Eldaran]

In a mythic campaign you're not supposed to give all monsters mythic ranks, the PCs are supposed to be special and powerful and only certain enemies should be mythic too. The book suggests you increase the CR of encounters by 1 for every 2 tiers the PCs have, but I find that's not generally enough. It depends on your party's class makeup, and what abilities they're taking, but 1 CR per tier is probably more accurate, especially if you increase the CR by adding more enemies rather than just singular enemies with high CR.

[Kol Korran]

Mythic changes the game considerably. I'm DMing the Wrath of the Righteous Paizo AP, which uses mythic rules. It makes the characters FAR more powerful, with the casters' ability to cast any spell on their list spontaneously the main offender- many challenges get far easier.

As Eldaran said, you'll need to upgrade the CR. His assumption is quite correct, but you'll need to adjust it to the party itself. If you're having problems, have the demigod offer some other alternative, like maybe a free feat, or maybe even upgrading one or two feats into mythic feats.

The mythic rule set is quite big and somewhat cumbersome. The two biggest problems for the DM are dealing with the sheer power and versatility of the PCs, and that there aren't really good rules into making monsters mythic, it is mostly left up to you to come up with mythic abilities (The examples on the site are really not enough). Unless the campaign's MAIN focus is the mythic theme, avoid it. It changes the game to a great extent.

[Selion]

It seems to me that first mythic tier is better than a level (1 path ability, 1 mythic ability, 1 mythic feat, some PFs, surge), after that the CR 1/2 can be applied, (free ability scores/feats, but no BAB, TS and gear improvement).
I made up my mind... just after this encounter they will be part of a few non mythic adventures, with just the boss with some mythic ranks, while in the end of the campaign the party will likely face god/planar issues, and i prefer the mythic progression over the old epic levels rules. I will stop players progression at level 20/MR 3.